The overall diameter for the front tire is 27.1", and the rear is 27.4", so if you can find an aftermarket spare with the correct bolt pattern, it should work, but make sure it fits in the trunk well. Ford does sell a spare tire kit, part # FR3Z-1K007-C, for the mustang. It has a disclaimer "not for cars originally equipped with summer only tires or 20" wheels. Our cars are equipped with summer only tire, but this would be better than nothing in a pinch.
And you are correct, there are some things fix-a-flat wont fix. I hit a pothole in the Bronx in '12 mustang and all the fix-a-flat in the world was not going to fill up a tire with a 10" gash in it. Luckily I was on the highway and got picked up by a tow truck, that was waiting near the pothole , he took me to a shop he knew just off the highway. The shop didn't have a garage, he just dropped me at a parking meter in front of the shop. In less than 15 min, and about $60 I had a reasonable used replacement tire on my car and I was on my way. I was lucky it could have been much worse.
Edited by Mach1Latus, 22 July 2016 - 08:10 PM.